Method and structure for thin film tandem photovoltaic cell

ABSTRACT

A tandem photovoltaic cell. The tandem photovoltaic cell includes a bifacial top cell and a bottom cell. The top bifacial cell includes a top first transparent conductive oxide material. A top window material underlies the top first transparent conductive oxide material. A first interface region is disposed between the top window material and the top first transparent conductive oxide material. The first interface region is substantially free from one or more entities from the top first transparent conductive oxide material diffused into the top window material. A top absorber material comprising a copper species, an indium species, and a sulfur species underlies the top window material. A top second transparent conductive oxide material underlies the top absorber material. A second interface region is disposed between the top second transparent conductive oxide material and the top absorber material. The bottom cell includes a bottom first transparent conductive oxide material. A bottom window material underlies the first bottom transparent conductive oxide material. A bottom absorber material underlies the bottom window material. A bottom electrode material underlies the bottom absorber material. The tandem photovoltaic cell further includes a coupling material free from a parasitic junction between the top cell and the bottom cell.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional application of U.S. Non-provisionalapplication Ser. No. 13/030,464, filed Feb. 18, 2011, which is adivisional of U.S. Non-provisional application Ser. No. 12/562,086,filed Sep. 17, 2009, which claims priority to U.S. ProvisionalApplication No. 61/101,641, filed Sep. 30, 2008, which is commonlyassigned and hereby incorporated by reference in its entirety herein forall purpose.

This application is related to Provisional Application No.: 61/101,642(Attorney Docket Number 026335-005800US) filed Sep. 30, 2008, commonlyassigned, and hereby incorporated by reference herein for all purpose.This application is also related to PCT Application No.: PCT/US09/46161(Attorney Docket Number 026335-002510PC) filed Jun. 3, 2009, commonlyassigned, and hereby incorporated by reference herein for all purpose.

BACKGROUND OF THE INVENTION

The present invention relates generally to photovoltaic materials andmanufacturing method. More particularly, the present invention providesa method and structure for a thin film tandem photovoltaic cells. Merelyby way of example, the present method and structure include absorbermaterials comprising copper indium disulfide species.

From the beginning of time, mankind has been challenged to find way ofharnessing energy. Energy comes in the forms such as petrochemical,hydroelectric, nuclear, wind, biomass, solar, and more primitive formssuch as wood and coal. Over the past century, modern civilization hasrelied upon petrochemical energy as an important energy source.Petrochemical energy includes gas and oil. Gas includes lighter formssuch as butane and propane, commonly used to heat homes and serve asfuel for cooking Gas also includes gasoline, diesel, and jet fuel,commonly used for transportation purposes. Heavier forms ofpetrochemicals can also be used to heat homes in some places.Unfortunately, the supply of petrochemical fuel is limited andessentially fixed based upon the amount available on the planet Earth.Additionally, as more people use petroleum products in growing amounts,it is rapidly becoming a scarce resource, which will eventually becomedepleted over time.

More recently, environmentally clean and renewable sources of energyhave been desired. An example of a clean source of energy ishydroelectric power. Hydroelectric power is derived from electricgenerators driven by the flow of water produced by dams such as theHoover Dam in Nevada. The electric power generated is used to power alarge portion of the city of Los Angeles in California. Clean andrenewable sources of energy also include wind, waves, biomass, and thelike. That is, windmills convert wind energy into more useful forms ofenergy such as electricity. Still other types of clean energy includesolar energy. Specific details of solar energy can be found throughoutthe present background and more particularly below.

Solar energy technology generally converts electromagnetic radiationfrom the sun to other useful forms of energy. These other forms ofenergy include thermal energy and electrical power. For electrical powerapplications, solar cells are often used. Although solar energy isenvironmentally clean and has been successful to a point, manylimitations remain to be resolved before it becomes widely usedthroughout the world. As an example, one type of solar cell usescrystalline materials, which are derived from semiconductor materialingots. These crystalline materials can be used to fabricateoptoelectronic devices that include photovoltaic and photodiode devicesthat convert electromagnetic radiation into electrical power. However,crystalline materials are often costly and difficult to make on a largescale. Additionally, devices made from such crystalline materials oftenhave low energy conversion efficiencies. Other types of solar cells use“thin film” technology to form a thin film of photosensitive material tobe used to convert electromagnetic radiation into electrical power.Similar limitations exist with the use of thin film technology in makingsolar cells. That is, efficiencies are often poor. Additionally, filmreliability is often poor and cannot be used for extensive periods oftime in conventional environmental applications. Often, thin films aredifficult to mechanically integrate with each other. These and otherlimitations of these conventional technologies can be found throughoutthe present specification and more particularly below.

From the above, it is seen that improved techniques for manufacturingphotovoltaic materials and resulting devices are desired.

BRIEF SUMMARY OF THE INVENTION

According to embodiments of the present invention, a method and astructure for forming a photovoltaic cell is provided. Moreparticularly, the present invention provides a method and structure forforming thin film tandem photovoltaic cell. Merely by way of example,embodiments according to the present invention have been implementedusing thin film semiconductor material. But it would be recognized thatembodiments according to the present invention can have a much broaderrange of applicability.

In a specific embodiment, a tandem photovoltaic cell is provided. Thetandem photovoltaic cell includes a top cell. The top cell is a bifacialcell in a specific embodiment. The top cell includes a top firsttransparent conductive oxide material. A top window material underliesthe top first transparent conductive oxide material. In a specificembodiment, the top cell includes a first interface region disposedbetween the top window material and the top first transparent conductiveoxide material. The first interface region is substantially free fromone or more entities from the top first transparent conductive oxidematerial being diffused into the top window material. The top cell alsoincludes a top absorber material underlying the top window material. Thetop absorber material comprise a copper species, an indium species, anda sulfur species in a specific embodiment. The top cell includes a topsecond transparent conductive oxide material underlying the top absorbermaterial and a second interface region disposed between the top secondtransparent conductive oxide material and the top absorber material. Thesecond interface region is substantially free from one or more entitiesfrom the top first transparent conductive oxide material being diffusedinto the top absorber material.

The tandem photovoltaic includes a bottom cell. The bottom cell includesa bottom first transparent conductive oxide material. A bottom windowmaterial underlies the first bottom transparent conductive oxidematerial. In a specific embodiment, a bottom absorber material isprovided underlying the bottom window material and a bottom electrodematerial is provided underlying the bottom absorber material. In aspecific embodiment, a coupling material is disposed between the topcell and the bottom cell. The coupling material is free from a parasiticjunction between the top cell and the bottom cell in a preferredembodiment.

In an alternative embodiment, an alternative tandem photovoltaic cell isprovided. The alternative tandem photovoltaic cell includes a top cell.The top cell is a bifacial cell in a specific embodiment. The top cellincludes a top first transparent conductive oxide material. A top windowmaterial underlies the top first transparent conductive oxide material.The top cell also includes a top absorber material underlying the topwindow material. The top absorber material comprise a copper species, anindium species, and a sulfur species in a specific embodiment. The topcell includes a top second transparent conductive oxide materialunderlying the top absorber material.

The alternative tandem photovoltaic includes a bottom cell. The bottomcell includes a bottom first transparent conductive oxide material. Abottom window material underlies the first bottom transparent conductiveoxide material. In a specific embodiment, a bottom absorber material isprovided underlying the bottom window material and a bottom electrodematerial is provided underlying the bottom absorber material. In aspecific embodiment, a coupling material is disposed between the topcell and the bottom cell. The coupling material is free from a parasiticjunction between the top cell and the bottom cell in a preferredembodiment.

In a specific embodiment, a method of forming a tandem photovoltaic cellis provided. The method includes forming a bifacial top cell. Thebifacial top cell is formed by providing a top first transparentconductive oxide material. A top window material is formed underlyingthe top first transparent conductive oxide material. In a specificembodiment, the method forms a first interface region between the topwindow material and the top first transparent conductive oxide material.In a specific embodiment, the first interface region is substantiallyfree diffusion of from one or more entities from the top firsttransparent conductive oxide material into the top window material. Themethod forms a top absorber material underlying the top window material.The top absorber material includes a copper species, an indium species,and a sulfur species in a specific embodiment. A top second transparentconductive oxide material is formed underlying the top absorbermaterial. The method includes forming a second interface region betweenthe top second transparent conductive oxide material and the topabsorber material for the bifacial top cell. The second interface regionis substantially free from one or more entities from the top firsttransparent conductive oxide material diffused into the top absorbermaterial in a preferred embodiment. The method includes forming a bottomcell. The bottom cell is formed by providing a bottom first transparentconductive oxide material and forming a bottom window materialunderlying the first bottom transparent conductive oxide material. Abottom absorber material is formed underlying the bottom windowmaterial. A bottom electrode material is formed underlying the bottomabsorber material to form the bottom cell. In a specific embodiment, themethod provides a coupling material disposed between the top bifacialcell and the bottom cell. Preferably, the coupling material provides fora junction free from parasitic potential between the top bifacial celland the bottom cell.

Many benefits can be achieved by ways of the embodiments according tothe present invention. For example, the thin film tandem photovoltaiccell can be fabricated using techniques without substantial modificationto the conventional equipment. Additionally the present thin film tandemphotovoltaic cell has an improved conversion efficiency compared to aconventional photovoltaic cell and provides a cost effective way toconvert sunlight into electric energy. Depending on the embodiment, oneor more of these benefits may be achieved. These and other benefits willbe described in more detailed throughout the present specification andparticularly below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of a tandem photovoltaic cell accordingto an embodiment of the present invention.

FIGS. 2 through 9 are schematic diagrams illustrating a method andstructure for forming a thin film photovoltaic device according to anembodiment of the present invention.

FIGS. 10 through 17 are schematic diagrams illustrating a method andstructure for forming a thin film photovoltaic device according to anembodiment of the present invention.

FIG. 18 is a simplified diagram illustrating a structure for a thin filmtandem photovoltaic cell according to an embodiment of the presentinvention.

FIG. 19 is a simplified diagram illustrating an alternative structurefor a thin film tandem photovoltaic cell according to an embodiment ofthe present invention.

FIG. 20 is a simplified diagram illustrating a test result for a thinfilm tandem photovoltaic cell according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

According to embodiments of the present, a method and a structure forforming a photovoltaic cell are provided. More particularly, embodimentsaccording to the present invention provide a method and structure forforming a thin film tandem photovoltaic cell. Merely by way of example,embodiments according to the present invention have been implementedusing thin film semiconductor material. But it would be recognized thatembodiments according to the present invention can have a much broaderrange of applicability.

FIG. 1 is a simplified diagram of a tandem photovoltaic cell accordingto an embodiment of the present invention. The diagram is merely anillustration and should not unduly limit the scope of the claims herein.One of ordinary skill in the art would recognize other variations,modifications, and alternatives. As an example, the tandem photovoltaiccell can also be described in U.S. Provisional No. 61/092,732, AttorneyDocket number 026335-003400US), commonly assigned, and herebyincorporated by reference herein. As shown, a four terminal tandemphotovoltaic cell device 100 is provided. The four terminal tandemphotovoltaic cell includes a lower cell 103 and an upper cell 101operably coupled to the lower cell. The terms “lower” and “upper” arenot intended to be limiting but should be construed by plain meaning byone of ordinary skill in the art. In general, the upper cell is closerto a source of electromagnetic radiation than the lower cell, whichreceives the electromagnetic radiation after traversing through theupper cell. Of course, there can be other variations, modifications, andalternatives.

In a specific embodiment, the lower cell includes a lower glasssubstrate material 119, e.g., a transparent glass material. The lowercell also includes a lower electrode layer made of a reflective materialoverlying the lower glass substrate material. The lower cell includes alower absorber layer overlying the lower electrode layer. As shown, theabsorber and electrode layer are illustrated by reference numeral 117.In a specific embodiment, the absorber layer is made of a semiconductormaterial having a band gap energy Eg in a range of about 1.2 eV to about2.2 eV and preferably in a range of about 1.6 eV to about 1.9 eV, butcan be others. In a specific embodiment, the lower cell includes a lowerwindow layer overlying the lower absorber layer and a lower transparentconductive oxide layer 115 overlying the lower window layer.

In a specific embodiment, the upper cell includes a p+ type transparentconductor layer 109 overlying the lower transparent conductive oxidelayer. In a preferred embodiment, the p+ type transparent conductorlayer is characterized by a sheet resistance of less than or equal toabout 10 Ohms/square centimeters and an optical transmission of 90percent and greater. In a specific embodiment, the upper cell has anupper p type absorber layer overlying the p+ type transparent conductorlayer. In a preferred embodiment, the p type conductor layer made of asemiconductor material has a band gap energy Eg in a range of about 1.2eV to about 2.2 eV and preferably in a range of about 1.6 eV to about1.9 eV, but can be others. The upper cell also has an upper n typewindow layer overlying the upper p type absorber layer. Referring againto FIG. 1, the window and absorber layer for the upper cell areillustrated by reference numeral 107. The upper cell also has an uppertransparent conductive oxide layer 105 overlying the upper n type windowlayer and an upper glass material (not shown) overlying the uppertransparent conductive oxide layer. Of course, there can be othervariations, modifications, and alternatives.

In a specific embodiment, the tandem photovoltaic cell includes fourterminals. The four terminals are defined by reference numerals 111,113, 121, and 123. Alternatively, the tandem photovoltaic cell can alsoinclude three terminals, which share a common electrode preferablyproximate to an interface region between the upper cell and the lowercell. In other embodiments, the multi junction cell can also include twoterminals, among others, depending upon the application. Examples ofother cell configurations are provided in U.S. Provisional PatentApplication No. 61/092,383, Attorney Docket No: 026335-001600US,commonly assigned and hereby incorporated by reference herein. Ofcourse, there can be other variations, modifications, and alternatives.Further details of the four terminal cell can be found throughout thepresent specification and more particularly below.

FIG. 2-17 are a schematic diagrams illustrating a method for forming atop cell for a thin film tandem photovoltaic device according to anembodiment of the present invention. These diagrams are merely examples,which should not unduly limit the claims herein. One skilled in the artwould recognize other variations, modifications, and alternatives. Asshown in FIG. 2, a substrate 110 is provided. In an embodiment, thesubstrate includes a surface region 112 and is held in a process stagewithin a process chamber (not shown). In another embodiment, thesubstrate is an optically transparent solid material. For example, thesubstrate can be a glass, quartz, fused silica, or a plastic, or metal,or foil, or semiconductor, or other composite materials. Depending uponthe embodiment, the substrate can be a single material, multiplematerials, which are layered, composites, or stacked, includingcombinations of these, and the like. Of course, there can be othervariations, modifications, and alternatives.

As shown in FIG. 3, the method includes forming a first electrode layer120 overlying the surface region of the substrate. The first electrodelayer can be formed using a suitable metal material such as molybdenum,or tungsten, but can be others. These other metal materials may includecopper, chromium, aluminum, nickel, platinum, or others. Such metalmaterial can be deposited using techniques such as sputtering,evaporation (e.g., electron beam), electro plating, combination of theseand the like in a specific embodiment. A thickness of the firstelectrode layer can range from about 100 nm to 2 micron, but can beothers. First electrode layer 120 is preferably characterized by aresistivity of about 10 Ohm/cm2 and less according to a specificembodiment. In a preferred embodiment, the electrode layer is providedby molybdenum. In a specific embodiment, the first electrode layer maybe provided using a transparent conductive oxide (TCO) material such asIn₂O₃:Sn (ITO), ZnO:Al (AZO), SnO2:F (TFO), but can be others. Ofcourse, there can be other variations, modifications and alternatives.

Referring to FIG. 4, the method for forming the thin film photovoltaiccell includes forming a copper layer 130 overlying the electrode layerformed. The copper layer can be formed using a sputtering process suchas a DC magnetron sputtering process in a specific embodiment. The DCmagnetron sputtering process may be provided at a deposition pressure ofabout 6.2 mTorr, controlled by using an inert gas such as argon. Suchpressure can be achieved using a gas flow rate of about 32 sccm. Thesputtering process can be perform at about room temperature withoutheating the substrate. Of course, minor heating of the substrate may beresulted due to the plasma generated during the deposition process.According to certain embodiments, a DC power in a range from 100 Wattsto 150 Watts, and preferably about 115 Watts may be used, depending onthe embodiment. A deposition time for a Cu layer of 330 nm thickness canbe about 6 minutes or more. Of course, the deposition condition can bevaried and modified according to a specific embodiment.

Depending on the embodiment, the method forms a barrier layer 125overlying the electrode layer to form an interface region between theelectrode layer and the copper layer. In a specific embodiment, theinterface region is maintained substantially free from a metal disulfidelayer having a semiconductor characteristic that is different from thecopper indium disulfide material during later processing steps.Depending upon the embodiment, the barrier layer has suitable conductivecharacteristics and can be reflective to allow electromagnetic radiationto reflect back or can also be transparent or the like. In a specificembodiment, the barrier layer is selected from platinum, titanium,chromium, or silver. Of course, there can be other variations,modifications, and alternatives.

As shown in FIG. 5, the method includes providing an indium (In) layer140 overlying the copper layer. In particular, the indium layer 140 isformed overlying the copper layer 130. The indium layer is depositedover the copper layer using a sputtering process. In one example, theindium layer is deposited using a DC magnetron sputtering process isunder a similar process condition for depositing the Cu layer. Thedeposition time for the indium layer may be shorter than that for Culayer. For example, 2 minutes and 45 seconds may be sufficient fordepositing an In layer of a thickness of about 410 nm according to aspecific embodiment. Other suitable deposition methods such aselectroplating or others may also be used depending on the embodiment.

In a specific embodiment, the copper layer and the indium layer form amultilayer structure for the thin film photovoltaic cell. In a specificembodiment, the copper layer and the indium layer are provided in acertain stoichiometry that forms a copper rich material having a copperto indium atomic ratio ranging from about 1.2:1 to about 2.0:1. In analternative embodiment, the copper to indium atomic ratio ranges fromabout 1.35:1 to about 1.60:1. In another embodiment, the copper toindium atomic ratio is selected to be 1.55:1. In a preferred embodiment,the copper to indium atomic ratio provides a copper rich film for thephotovoltaic cell. In another specific embodiment, the indium layer isdeposited overlying the electrode layer prior to the deposition of thecopper layer. Of course there can be other variations, modifications,and alternatives.

As shown in FIG. 5, the multilayered structure 150 comprising at leastan indium layer and a copper layer is subjected to a thermal treatmentprocess 200 in an sulfur species 210 bearing environment. The thermaltreatment process uses a rapid thermal process while the multilayerstructure is subjected to the sulfur bearing species. In a specificembodiment, the rapid thermal process uses a temperature ramp rateranging from about 10 Degrees Celsius/second to about 50 DegreesCelsius/second to a final temperature ranging from about 400 DegreesCelsius to about 600 Degrees Celsius. In a specific embodiment, thethermal treatment process further maintains at the final temperature fora dwell time ranging from about 1 minute to about 10 minutes, but can beothers. The thermal treatment process also includes a temperature rampdown in an inert ambient or other suitable environment. The inertambient can be provided using gases such as nitrogen, argon, helium, andothers, which stops reaction to alloy the metal material with the sulfurspecies. Further details of the temperature ramp process is describedthroughout the present specification and more particularly below.

In a specific embodiment, the sulfur bearing species can be appliedusing a suitable technique. In an example, the sulfur bearing speciesare in a fluid phase. As an example, the sulfur can be provided in asolution, which has dissolved Na₂S, CS₂, (NR₄)₂S, thiosulfate, andothers. Such fluid based sulfur species can be applied overlying one ormore surfaces of the multilayered copper/indium structure according to aspecific embodiment. In another example, the sulfur bearing species 210is provided by hydrogen sulfide gas or other like gas. In otherembodiments, the sulfur can be provided in a solid phase, for exampleelemental sulfur. In a specific embodiment, elemental sulfur can beheated and allowed to vaporize into a gas phase, e.g., S_(n). andallowed to react with the indium/copper layers. Other sulfur bearingspecies may also be used depending on the embodiment. Taking hydrogensulfide gas as the sulfur bearing species as an example. The hydrogensulfide gas can be provided using one or more entry valves with flowrate control into a process chamber. Any of these application techniquesand other combinations of techniques can also be used. The processchamber may be equipped with one or more pumps to control processpressure. Depending on the embodiment, a layer of sulfur material may beprovided overlying the multilayer structure comprising the copper layerand the indium layer. The layer of sulfur material can be provided as apatterned layer in a specific embodiment. In other embodiment, sulfurmaterial may be provided in a slurry, a powder, a solid, a paste, a gas,any combination of these, or other suitable form. Of course, there canbe other variations, modifications, and alternatives.

Referring again to FIG. 6, the thermal treatment process cause areaction between copper indium material within the multilayeredstructure and the sulfur bearing species 210, thereby forming a layer ofcopper indium disulfide thin film material 220. In one example, thecopper indium disulfide material is formed by incorporating sulfur ionsand/or atoms evaporated or decomposed from the sulfur bearing speciesinto the multilayered structure with indium atoms and copper atomsmutually diffused therein. In a specific embodiment, the thermaltreatment process results in a formation of a cap layer overlying thecopper indium disulfide material. The cap layer comprises a thickness ofsubstantially copper sulfide material 221 substantially free of indiumatoms. The copper sulfide material 221 includes a surface region 225. Ina specific embodiment, the formation of the copper sulfide cap layer isunder a Cu-rich conditions for the Cu—In bearing multilayered structure150. Depending on the embodiment, the thickness of the copper sulfidematerial is in an order of about five to ten nanometers and greaterdepending on the multilayered structure. In a specific embodiment,thermal treatment process allows the first electrode layer using a TCOmaterial to maintain at a sheet resistance of less than or equal toabout 10 Ohms per square centimeters and an optical transmission of 90percent and greater after the copper indium disulfide thin film materialis formed. Of course, there can be other variations, modifications, andalternatives.

As shown in FIG. 7, copper sulfide material 221 is subjected to a dipprocess 300. The copper sulfide material overlies copper indiumdisulfide thin film 220. The dip process is performed by exposing thesurface region of the copper sulfide material to about a solutioncomprising a 10% by weight of potassium cyanide 310 according to aspecific embodiment. Potassium cyanide solution provides an etchingprocess to selectively remove copper sulfide material 221 from thecopper indium disulfide material surface exposing a surface region 228of underlying copper indium disulfide material according to a specificembodiment. In a preferred embodiment, the etch process has aselectivity of about 1:100 or more between copper sulfide and copperindium disulfide. Other etching species can be used depending on theembodiment. In a specific embodiment, the etching species can behydrogen peroxide. In other embodiments, other etching techniquesincluding electro-chemical etching, plasma etching, sputter-etching, orany combination of these may be used. In a specific embodiment, thecopper sulfide material can be mechanically removed, chemically removed,electrically removed, or any combination of these, and others. In aspecific embodiment, the absorber layer made of copper indium disulfidecan have a thickness ranging from about one micron to about 10 microns,but can be others. Of course, there can be other variations,modifications, and alternatives.

In a specific embodiment, the copper indium disulfide film has a p typeimpurity characteristics. In certain embodiments, the copper indiumdisulfide material is further subjected to a doping process to adjust ap-type impurity density therein to optimize an I-V characteristic of thehigh efficiency thin film photovoltaic devices. For example, the copperindium disulfide material may be doped using an aluminum species. Inanother example, the copper indium disulfide material can be intermixedwith a copper indium aluminum disulfide material to form the absorberlayer. Of course, there can be other variations, modifications, andalternatives.

Referring again to FIG. 8, the method includes forming a window layer310 overlying the copper indium disulfide material, which has a p-typeimpurity characteristics. The window layer can be selected from a groupof materials consisting of a cadmium sulfide (CdS), a zinc sulfide(ZnS), zinc selenium (ZnSe), zinc oxide (ZnO), zinc magnesium oxide(ZnMgO), or others. These material may be doped with a suitableimpurities to provide for a n+ type impurity characteristic. The windowlayer and the absorber layer forms an interface region for a PN-junctionassociated with a photovoltaic cell. The window layer is heavily dopedto form a n+-type semiconductor layer. In one example, indium speciesare used as the doping material for a CdS window layer to causeformation of the n+-type characteristic associated with the windowlayer. In certain embodiments, ZnO may be used as the window layer. ZnOcan be doped with an aluminum species to provide for the n+ impuritycharacteristics. Depending on the material used, the window layer canrange from about 200 nanometers to about 500 nanometers. Of course,there can be other variations, modifications, and alternative.

As shown in FIG. 9, a conductive layer 330 is form overlying a portionof a surface region of the window layer. Conductor layer 330 provides atop electrode layer for the thin film photovoltaic cell. In oneembodiment, conductive layer 330 is a transparent conductive oxide(TCO). For example, the transparent conductive oxide can be selectedfrom a group consisting of In₂0₃:Sn (ITO), ZnO:Al (AZO), SnO2:F (TFO),but can be others. In a specific embodiment, the TCO layer is patternedto maximize the efficiency of the thin film photovoltaic devices. Incertain embodiments, the TCO layer can also function as a window layer,which eliminates the need of a separate window layer. Of course therecan be other variations, modifications, and alternatives.

FIG. 10 through 17 are simplified diagrams illustrating a method to forma photovoltaic cell in a superstrate configuration for the thin filmtandem photovoltaic cell according to an alternative embodiment of thepresent invention. These diagrams are merely examples and should notunduly limit the scope of the claims herein. One skilled in the artwould recognize other variations, modifications, and alternatives. Asshown in FIG. 10, a substrate 1010 is provided. In an embodiment, thesubstrate includes a surface region 1012 and is held in a process stagewithin a process chamber (not shown). In a specific embodiment, thetransparent substrate is an optically transparent solid material. Forexample, the optically transparent solid material can be glass, quartz,fused silica, or a polymer material. Other material such as metal, orfoil, or semiconductor, or other composite materials may also be used inother embodiments. Depending upon the embodiment, the substrate can be asingle material, multiple materials, which are layered, composites, orstacked, including combinations of these, and the like. Of course therecan be other variations, modifications, and alternatives.

As shown in FIG. 11, the method includes forming a first electrode layer1102 including a electrode surface region overlying the surface regionof the substrate. The first electrode layer is preferably made of atransparent conductive oxide, commonly called TCO. For example, thetransparent conductive oxide can be selected from a group consisting ofIn₂O₃:Sn (ITO), ZnO:Al (AZO), SnO2:F (TFO), but can be others. In aspecific embodiment, the TCO layer is patterned to maximize theefficiency of the thin film photovoltaic devices. A thickness of theelectrode layer can range from about 100 nm to 2 micron, but can beothers. Electrode layer 120 is preferably characterized by a resistivityof less than about 10 Ohm/cm² according to a specific embodiment. Ofcourse there can be other variations, modifications, and alternatives.

In a specific embodiment, the method includes forming a window layer1202 overlying the first electrode layer as shown in FIG. 12. The windowlayer can be selected from a group of materials consisting of a cadmiumsulfide (CdS), a zinc sulfide (ZnS), zinc selenium (ZnSe), zinc oxide(ZnO), zinc magnesium oxide (ZnMgO), or others. These material may bedoped with a suitable impurities to provide for a n+ type impuritycharacteristic. In one example, indium species are used as the dopingmaterial for a CdS window layer to cause formation of the n+-typecharacteristic associated with the window layer. In certain embodiments,ZnO may be used as the window layer. ZnO can be doped with an aluminumspecies to provide for the n+ impurity characteristics. Depending on thematerial used, the window layer can range from about 200 nanometers toabout 500 nanometers. Of course, there can be other variations,modifications, and alternative.

Referring to FIG. 13, the method includes providing a copper layer 1302overlying the window layer. The copper layer can be formed using asputtering process such as a DC magnetron sputtering process in aspecific embodiment. The DC magnetron sputtering process may be providedat a deposition pressure of about 6.2 mTorr, controlled by using aninert gas such as argon. Such pressure can be achieved using a gas flowrate of about 32 sccm. The sputtering process can be perform at aboutroom temperature without heating the substrate. Of course, minor heatingof the substrate may be resulted due to the plasma generated during thedeposition process. According to certain embodiments, a DC power in arange from 100 Watts to 150 Watts, and preferably about 115 Watts may beused, depending on the embodiment. As merely an example, a depositiontime for a Cu layer of 330 nm thickness can be about 6 minutes or more.Of course, the deposition condition can be varied and modified accordingto a specific embodiment.

As shown in FIG. 14, the method includes providing an indium (In) layer1402 overlying the copper layer. The indium layer is deposited over thecopper layer using a sputtering process in a specific embodiment. In oneexample, the indium layer is deposited using a DC magnetron sputteringprocess is under a similar process condition for depositing the Culayer. The deposition time for the indium layer may be shorter than thatfor Cu layer. For example, 2 minutes and 45 seconds may be sufficientfor depositing an In layer of a thickness of about 410 nm according to aspecific embodiment. Other suitable deposition methods such aselectroplating or others may also be used depending on the embodiment.

In a specific embodiment, the copper layer and the indium layer form amultilayer structure 1404 for the thin film photovoltaic cell. In aspecific embodiment, the copper layer and the indium layer are providedin a certain stoichiometry that forms a copper rich material. In aspecific embodiment, the copper rich material can have a copper toindium atomic ratio ranging from about 1.2:1 to about 2.0:1. In analternative embodiment, the copper to indium atomic ratio ranges fromabout 1.35:1 to about 1.60:1. In another embodiment, the copper toindium atomic ratio is selected to be 1.55:1. In a preferred embodiment,the copper to indium atomic ratio provides a copper rich film for thephotovoltaic cell. In another specific embodiment, the indium layer isdeposited overlying the electrode layer prior to the deposition of thecopper layer. Of course there can be other variations, modifications,and alternatives.

As shown in FIG. 15, the multilayered structure comprising at least anindium layer and a copper layer is subjected to a thermal treatmentprocess 1502 in an sulfur species 1504 bearing environment. The thermaltreatment process uses a rapid thermal process while the multilayerstructure is subjected to the sulfur bearing species. In a specificembodiment, the rapid thermal process uses a temperature ramp rateranging from about 10 Degrees Celsius/second to about 50 DegreesCelsius/second to a final temperature ranging from about 400 DegreesCelsius to about 600 Degrees Celsius. In a specific embodiment, thethermal treatment process further maintains at the final temperature fora dwell time ranging from about 1 minute to about 10 minutes, but can beothers. The thermal treatment process also include a temperature rampdown in an inert ambient or other suitable environment that can stop thereaction of formation of the alloy material in a specific embodiment.The inert ambient can be provided using gases such as nitrogen, argon,helium, and others. Further details of the temperature ramp process isdescribed throughout the present specification and more particularlybelow.

In a specific embodiment, the sulfur bearing species can be appliedusing a suitable technique. In an example, the sulfur bearing speciesare in a fluid phase. As an example, the sulfur can be provided in asolution, which has dissolved Na₂S, CS₂, (NR₄)₂S, thiosulfate, andothers. Such fluid based sulfur species can be applied overlying one ormore surfaces of the multilayered copper/indium structure according to aspecific embodiment. In another example, the sulfur bearing species 210is provided by hydrogen sulfide gas or other like gas. In otherembodiments, the sulfur can be provided in a solid phase, for exampleelemental sulfur. In a specific embodiment, elemental sulfur can beheated and allowed to vaporize into a gas phase, e.g., S_(n). andallowed to react with the indium/copper layers. Other sulfur bearingspecies may also be used depending on the embodiment. Taking hydrogensulfide gas as the sulfur bearing species as an example. The hydrogensulfide gas can be provided using one or more entry valves with flowrate control into a process chamber. Any of these application techniquesand other combinations of techniques can also be used. The processchamber may be equipped with one or more pumps to control processpressure. Depending on the embodiment, a layer of sulfur material may beprovided overlying the multilayer structure comprising the copper layerand the indium layer. The layer of sulfur material can be provided as apatterned layer in a specific embodiment. In other embodiment, sulfurmaterial may be provided in a slurry, a powder, a solid, a paste, a gas,any combination of these, or other suitable form. Of course, there canbe other variations, modifications, and alternatives.

In a specific embodiment, the thermal treatment process maintains theabsorber layer substantially free from species that may diffuse from thewindow layer and/or the transparent conductive oxide layer. The methodalso eliminates using a thick window layer to protect the transparentconductive oxide layer from diffusion of species from the absorberlayer. The method provides a photovoltaic cell that can have aconversion efficiency greater than about 8 percent or greater than 10percent, and others. Of course, there can be other variations,modifications, and alternatives.

Referring again to FIG. 15, the thermal treatment process causes areaction between copper and indium materials within the multilayeredstructure and the sulfur bearing species., thereby forming a layer ofcopper indium disulfide thin film material 1506. In one example, thecopper indium disulfide thin film material is formed by incorporatingsulfur ions and/or atoms evaporated or decomposed from the sulfurbearing species into the multilayered structure with indium atoms andcopper atoms mutually diffused therein. In a specific embodiment, thethermal treatment process results in a formation of a cap layeroverlying the copper indium disulfide material. The cap layer comprisesa thickness of substantially copper sulfide material 1508 substantiallyfree of indium atoms. The copper sulfide material includes a surfaceregion 1510. In a specific embodiment, the formation of the coppersulfide cap layer is under a Cu-rich conditions for the Cu—In bearingmultilayered structure. Depending on the embodiment, the thickness ofthe copper sulfide material is in an order of about five to tennanometers and greater depending on the multilayered structure. In aspecific embodiment, the thermal treatment process allows the firstelectrode layer to maintain at a sheet resistance of less than or equalto about 10 Ohms per square centimeters and an optical transmission of90 percent and greater after the copper indium disulfide thin filmmaterial is formed. Of course, there can be other variations,modifications, and alternatives.

As shown in FIG. 16, the copper sulfide material is subjected to a dipprocess 1602. The dip process is performed by exposing the surfaceregion of the copper sulfide material to a solution 1604 comprisingpotassium cyanide as an etching species at a concentration of about a10% by weight according to a specific embodiment. Potassium cyanidesolution provides an etching process to selectively remove coppersulfide material from the copper indium disulfide material surfaceexposing a surface region 1606 of underlying copper indium disulfidematerial according to a specific embodiment. In a preferred embodiment,the etching process has a selectivity of about 1:100 or more betweencopper sulfide and copper indium disulfide. Other etching species can beused depending on the embodiment. In a specific embodiment, the etchingspecies can be hydrogen peroxide. In other embodiments, other etchingtechniques including electro-chemical etching, plasma etching,sputter-etching, or any combination of these may be used. In a specificembodiment, the copper sulfide material can be mechanically removed,chemically removed, electrically removed, or any combination of these,and others In a specific embodiment, the absorber layer made of copperindium disulfide can have a thickness ranging from about one micron toabout 10 microns, but can be others. Of course, there can be othervariations, modifications, and alternatives.

In a specific embodiment, the copper indium disulfide film has a p typeimpurity characteristics and provide for an absorber layer for the thinfilm photovoltaic cell. In certain embodiments, the copper indiumdisulfide material is further subjected to a doping process to adjust ap-type impurity density therein to optimize an I-V characteristic of thehigh efficiency thin film photovoltaic devices. For example, the copperindium disulfide material may be doped using an aluminum species. Inanother example, the copper indium disulfide material can be intermixedwith a copper indium aluminum disulfide material to form the absorberlayer. The window layer and the absorber layer forms an interface regionfor a PN junction associated with a photovoltaic cell. Of course, therecan be other variations, modifications, and alternatives

As shown in FIG. 17, the method forms a second electrode layer 1702overlying the absorber layer. The second electrode layer can be atransparent conductive oxide (TCO) in a specific embodiment. Forexample, the transparent conductive oxide can be selected from a groupconsisting of In₂O₃:Sn (ITO), ZnO:Al (AZO), SnO2:F (TFO), but can beothers. In certain embodiments, the second electrode layer may beprovided using a metal material such as tungsten, gold, silver, copperor others. In other embodiments, the second electrode layer can bereflective to reflect electromagnetic radiation back to the photovoltaiccell and improves the conversion efficiency of the photovoltaic cell. Ofcourse there can be other variations, modifications, and alternatives.

In a specific embodiment , the method includes coupling the top cell andthe bottom cell to form the thin film tandem cell as illustrated inFIG. 1. In a specific embodiment, the top cell and the bottom cell maybe coupled using a suitable optical transparent material such as ethylvinyl acetate but can be others depending on the application. Of course,there can be other variations, modifications, and alternatives. In aspecific embodiment, other substrate configurations are described below.

FIGS. 18 and 19 are simplified diagrams illustrating structures of atandem photovoltaic cell according to embodiments of the presentinvention. As shown in FIG. 18, a structure for a tandem photovoltaiccell is provided includes a top cell 1802 and a bottom cell 1804. Thetop cell can be a bifacial cell in a specific embodiment. The top cellincludes a top first transparent conductive oxide material 1806. The topfirst transparent conductive oxide (TCO) material can include materialssuch as indium tin oxide (ITO), aluminum doped zinc oxide (ZnO:Al),Fluorine doped tin oxide (SnO₂:F), or others. The top cell includes atop window material 1802 underlying the first transparent conductiveoxide material. In a specific embodiment, the top window material usesan n type semiconductor thin film material such as cadmium sulfide(CdS), zinc sulfide (ZnS), zinc selenium (ZnSe), zinc oxide (ZnO), orzinc magnesium oxide (ZnMgO), but can be others. The n typesemiconductor material is preferably heavily doped to have a n+ typeimpurity characteristic. Of course there can be other variations,modifications, and alternatives.

Referring again to FIG. 18, the top cell includes a first interfaceregion 1810 disposed between the top first transparent conductive oxideand the top window material. The first interface region is maintainedsubstantially free from one or more entities from a diffusion of the topfirst transparent conductive oxide material into the top windowmaterial. Depending upon the embodiment, the barrier layer has suitableconductive characteristics and can be optically transparent. Of course,there can be other variations, modifications, and alternatives.

The top cell includes a top absorber material 1812 underlying the topwindow layer. The top absorber material has a p type impuritycharacteristic in a preferred embodiment. In a specific embodiment, thetop absorber material comprises at least a top absorber materialunderlying the top window material, the top absorber material comprisinga copper species, an indium species, and a sulfur species in a specificembodiment. In certain embodiment, the top absorber material can includea copper indium disulfide thin film material, copper indium aluminumdisulfide thin film material, a copper indium gallium disulfidematerial, or a (Ag,Cu)(In,Ga)S₂ material, but can also be others,depending on the application. Of course, there can be other variations,modifications, and alternatives.

As shown in FIG. 18, the top cell includes a top second transparentconductive oxide material 1814 underlying the top absorber material. Thesecond top second transparent conductive oxide material can includematerials such as indium tin oxide (ITO), aluminum doped zinc oxide(ZnO:Al), Fluorine doped tin oxide (SnO₂:F), or others, depending on theembodiment.

In a specific embodiment, the top cell includes a second interfaceregion 1816 dispose between the top second transparent conductive oxideand the top absorber material. The second interface region is maintainedsubstantially free from one or more entities from a diffusion of the topsecond TCO material into the top absorber material. Depending upon theembodiment, the barrier layer has suitable conductive characteristicsand can be optically transparent. Of course, there can be othervariations, modifications, and alternatives.

Referring to again to FIG. 18. Bottom cell 1804 includes a bottom firsttransparent conductive oxide material 1818. The bottom first transparentconductive oxide (TCO) material can include materials such as indium tinoxide (ITO), aluminum doped zinc oxide (ZnO:Al), Fluorine doped tinoxide (SnO₂:F), or others. The bottom cell includes a bottom windowmaterial 1820 underlying the bottom first transparent conductive oxidematerial. In a specific embodiment, the bottom window material uses an ntype semiconductor thin film material such as cadmium sulfide (CdS),zinc sulfide (ZnS), zinc selenium (ZnSe), zinc oxide (ZnO), or zincmagnesium oxide (ZnMgO), but can be others. The n type semiconductormaterial is preferably heavily doped to have a n+ type impuritycharacteristic. Of course there can be other variations, modifications,and alternatives.

The bottom cell includes a bottom absorber material 1822 underlying thewindow layer. The bottom absorber material has a p type impuritycharacteristic in a preferred embodiment. In a specific embodiment, thebottom absorber material comprises at least a copper species, an indiumspecies, and a sulfur species in a specific embodiment. In certainembodiment, the bottom absorber material can include a copper indiumdisulfide thin film material, copper indium aluminum disulfide thin filmmaterial, or a copper indium gallium disulfide material, but can also beothers, depending on the application. In other embodiments, the bottomabsorber material can be Cu₂SnS₃; Cu(In,Ga)Se₂; CuInSe₂; or FeSi₂. Ofcourse, there can be other variations, modifications, and alternatives.

As shown in FIG. 18, the bottom cell includes a bottom electrodematerial 1824 underlying the bottom absorber material. The bottomelectrode material can include a transparent conductive oxide materialsuch as indium tin oxide (ITO), aluminum doped zinc oxide (ZnO:Al),Fluorine doped tin oxide (SnO₂:F), and the like. The bottom electrodematerial may also include a metal material such as copper, nickel, gold,tungsten and others, depending on the embodiment. In a specificembodiment the bottom electrode material is provided using a molybdenummaterial. Of course, there can be other variations, modifications, andalternatives.

In a specific embodiment, the tandem thin film photovoltaic cellincludes a coupling material 1826 provided between the top cell and thebottom cell. The coupling material is preferably an opticallytransparent material and free from a parasitic junction between the topcell and the bottom cell in a specific embodiment. In a specificembodiment, the optically transparent material can include material suchas ethyl vinyl acetate and the like. Of course, there can be othervariations, modifications, and alternatives.

Depending on the embodiment, the first interface region and the secondinterface region for the top cell can be optional. That is, the top cellis configured to have the top window material to underlie the top firsttransparent conductive oxide and the top second transparent conductiveoxide to underlie the top absorber material as shown in FIG. 19. Ofcourse there can be other variations, modifications, and alternatives.

FIG. 20 is an exemplary solar cell I-V characteristics plot measuredfrom a copper indium disulfide based thin film photovoltaic cellaccording to an embodiment of the present invention. The diagram ismerely an example, which should not unduly limit the claims herein. Oneskilled in the art would recognize other variations, modifications, andalternatives. As shown in FIG. 20, a current density of a highefficiency copper indium disulfide thin film photovoltaic cell madeaccording to an embodiment of the present invention is plotted against abias voltage. Further details of the thin film photovoltaic cell and theexperimental results are described in PCT Application No.:PCT/US09/46161 (Attorney Docket Number 026335-002510PC) filed Jun. 3,2009, commonly assigned, and hereby incorporate by reference. The curveintersects the y-axis with a short circuit current value at about 0.0235A/cm² and intersects a zero current line with a bias at about 0.69 V.The corresponding photovoltaic cell has an absorber layer made fromcopper indium disulfide thin film according to an embodiment of thepresent invention. In particular, the absorber layer is about 1.5 μm inthickness and an atomic ratio of Cu:In at about 1.5:1. Based on standardformula, a cell conversion efficiency η can be estimated:

$\eta = \frac{J_{SC} \cdot V_{OC} \cdot {FF}}{P_{in}\left( {{AM}\; 1.5} \right)}$

where J_(SC) is the short circuit current density of the cell, V_(OC) isthe open circuit bias voltage applied, FF is the so-called fill factordefined as the ratio of the maximum power point divided by the opencircuit voltage (Voc) and the short circuit current (J_(SC)). The inputlight irradiance (P_(in), in W/m²) under standard test conditions [i.e.,STC that specifies a temperature of 25° C. and an irradiance of 1000W/m2 with an air mass 1.5 (AM1.5) spectrum.] and the surface area of thesolar cell (in m²). Thus, a 10.4% efficiency can be accurately estimatedfor this particular cell made from a method according to embodiments ofthe present invention. In a specific embodiment, the bandgap is about1.45 eV to 1.5 eV. Of course, there can be other variations,modifications, and alternatives.

Although the above has been illustrated according to specificembodiments, there can be other modifications, alternatives, andvariations. For example, the method can be used to fabricate aphotovoltaic cell that has an absorber layer that forms using a hightemperature process. Although the above has been described in terms of aspecific absorber material, other absorber materials such as Cu(InAl)S₂Cu(InGa)S₂, Cu₂SnS, Cu₂ZnSnS₄, SnS, any combinations of these, andothers can be used. It is understood that the examples and embodimentsdescribed herein are for illustrative purposes only and that variousmodifications or changes in light thereof will be suggested to personsskilled in the art and are to be included within the spirit and purviewof this application and scope of the appended claims.

What is claimed is:
 1. A method for fabricating a tandem photovoltaic cell, comprising: forming a first cell comprising: forming a first transparent conductive oxide layer; forming a first absorber layer over the first transparent conductive oxide, the first absorber layer comprising a copper species, an indium species, and a sulfur species; forming a first window layer over the first absorber layer; and forming a second transparent conductive oxide layer over the first window layer; forming a second cell comprising: forming a first electrode layer; forming a second absorber material over the first electrode layer; forming a second window layer over the second absorber layer; and forming a third transparent conductive oxide layer over the second window layer; forming a coupling layer overlying the second transparent conductive oxide layer; and coupling the first cell with the second cell.
 2. The method of claim 1 wherein the first transparent conductive oxide comprises indium tin oxide (ITO), aluminum doped zinc oxide (ZnO:Al), or Fluorine doped tin oxide (SnO₂:F).
 3. The method of claim 1 wherein the first window material comprises cadmium sulfide (CdS), zinc sulfide (ZnS), zinc selenium (ZnSe), zinc oxide (ZnO), or zinc magnesium oxide (ZnMgO).
 4. The method of claim 1 wherein the first absorber material comprises copper indium disulfide (CIS), copper indium aluminum disulfide, copper indium gallium disulfide (CIGS), or (Ag,Cu)(In,Ga)S2.
 5. The method of claim 1 wherein the second transparent conductive oxide material comprises cadmium sulfide (CdS), zinc sulfide (ZnS), zinc selenium (ZnSe), zinc oxide (ZnO), or zinc magnesium oxide (ZnMgO).
 6. The method of claim 1 wherein the third transparent conductive oxide material comprises cadmium sulfide (CdS), zinc sulfide (ZnS), zinc selenium (ZnSe), zinc oxide (ZnO), or zinc magnesium oxide (ZnMgO).
 7. The method of claim 1 wherein the second window material comprises cadmium sulfide (CdS), zinc sulfide (ZnS), zinc selenium (ZnSe), zinc oxide (ZnO), or zinc magnesium oxide (ZnMgO).
 8. The method of claim 1 wherein the second electrode material comprises a transparent conductive oxide material or a metal material.
 9. The method of claim 1 wherein the second absorber material comprises a copper indium disulfide thin film material, a copper indium aluminum disulfide thin film material, or a copper indium gallium disulfide material.
 10. The method of claim 1 wherein the second absorber material comprises Cu₂SnS₃; Cu(In,Ga)Se₂; CuInSe₂; or FeSi₂.
 11. The method of claim 1 further comprising subjecting the first absorber material to a thermal treatment process, wherein the thermal treatment process includes increasing the temperature with a temperature ramp rate ranging from about 10 degrees Celsius/second to about 50 degrees Celsius/second.
 12. The method of claim 11 wherein the thermal treatment process is conducted in a sulfur-containing environment.
 13. The method of claim 1, wherein forming the first cell further includes: forming a first interface region between the first transparent conductive oxide layer and the first absorber layer; and forming a second interface region between the second transparent conductive oxide material and the first window layer, the second interface region being substantially free from one or more entities from the second transparent conductive oxide layer and the first window layer.
 14. The method of claim 13 wherein the second interface region comprises at least one material selected from the group consisting of platinum, chromium, and silver.
 15. The method of claim 1 wherein the coupling layer comprises an optically transparent material.
 16. The method of claim 1 wherein the coupling layer comprises ethyl vinyl acetate.
 17. A method for fabricating a tandem photovoltaic cell comprising: providing a substrate having a top surface and a bottom surface; forming a first electrode layer over the top surface of the substrate; forming a first absorber layer over the first electrode layer; forming a first window layer over the first absorber layer; forming a second electrode layer over the first window layer; forming a coupling layer over the second electrode layer; forming a third electrode layer over the coupling layer; forming a second absorber layer over the third electrode layer; forming a second window layer over the second absorber layer; and forming a fourth electrode layer over the second window layer.
 18. The method of claim 17, further comprising: forming a first interface region between the third electrode layer and the second absorber material, the first interface region being substantially free from one or more entities from the third electrode layer being and the second absorber layer; and forming a second interface region between the fourth electrode layer and the second window material, the second interface region being substantially free from one or more entities from the fourth electrode layer and the second window material.
 19. The method of claim 17, wherein the coupling layer comprises ethyl vinyl acetate. 